1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine and a printer, and in particular to a transferring member of an in-line type image forming apparatus utilizing an electrophotographic method.
2. Related Background Art
In recent years, an image forming apparatus such as an electrophotographic apparatus has been developed which is capable of realizing high speed and high performance and capable of forming a color image, and various types of printers are on the market.
Among these printers, there is an in-line type image forming apparatus that multiply transfers a plurality of colors of toner images formed on a plurality of stations to a sheet one after another while conveying a sheet as a recording medium by conveying means of a belt shape. This in-line type image forming apparatus is considered to become a main color printer because it can form a color image at a high speed.
An in-line type image forming apparatus is classified into an image forming apparatus of a type that multiply transfers an image on an intermediate transferring member and a sheet attracting type image forming apparatus that attracts a sheet on a transfer belt to multiply transfer an image on the sheet. It is advantageous to employ the sheet attracting type image forming apparatus having less system components in order to realize miniaturization of an apparatus and reduce costs.
Recently, from the viewpoint of realizing high performance of a printer, diversification of recording media, so that recording materials of various sizes and thicknesses (basis weights), light transmittable (permeable) resin for projection by an overhead projector (OHT) and the like can be used, and further the necessity of double-side printing and the like are required more than ever.
In addition, from the standpoint of an environment in which a printer is used, it is desired that a favorable output image be realized not only in an office that is typically fully equipped with air-conditioning but also in various environments such as in a general office and at an individual home.
Therefore, higher performance is required of a printer from the viewpoint of media flexibility and an environment of use.
However, the sheet attracting type in-line apparatus that attracts a sheet on a transfer belt and multiply transfers an image on the sheet has a problem in that it is susceptible to an influence of an environment where it is placed or a type of a sheet. This is because the sheet attracting type in-line apparatus is required to transfer an image on an sheet having an unstable element of resistance and to attract the sheet on an object being a transfer belt to transfer an image four times thereon.
Extremely high transfer voltage is required in order to make a transfer current to flow particularly at the time of automatic double-side printing when a sheet, which has been subject to fixing once and from which moisture has been evaporated to have high resistance, is fed again, or at the time of an OHT mode for printing on a transparent film that is insulating in the thickness direction.
In addition, since a sheet or a transfer belt receives a transfer charge for applying charge-up when transfer is received at an upstream station, an in-line method apparatus using a transfer belt requires a higher transfer voltage if transfer is received at more downstream station.
In a transfer section, a discharge is generated among a photosensitive body (OPC), a sheet, a transfer belt and a transfer material to transfer toner and give a charge to the sheet. If a transfer voltage is high, a charge defect or the like of a photosensitive body occurs due to an excessive discharge, an abnormal discharge, scattering of toner or transfer charge among them.
That is, if there is a leak site or the like between the transfer material and the transfer belt, image scattering occurs because a discharge is concentrated there. More specifically, a discharge mark of a stripe shape or a polka dot shape occurs on an image. This occurs markedly if there is a localized low resistance region on a material surface, and is caused by an extreme current flowing to the low resistance region.
In addition, if a transfer material surface is even, a discharge threshold value between the transferring member and the transfer belt increases and a voltage between gaps in a separation step at a transfer nip exit becomes higher, thus a discharge amount of one discharge becomes larger. Therefore, an image defect tends to occur.
In addition, in a reversal developing system, charge given to a photosensitive body varies according to a difference of electric potential contrasts viewed from a transferring member with respect to a dark portion potential and a light portion potential on a photosensitive body. A transfer charge with a polarity opposite that of a charged electric potential of the photosensitive body is given to a dark portion potential section where a transfer contrast is greater. Thus, there is also a problem such as a charge defect and a drum ghost due to an insufficient photosensitive body electric potential at the time of the next image formation.
The present invention has been devised in view of the above problems, and it is an object of the present invention to provide an image forming apparatus that restrains change in a transfer current and prevents an image defect.
It is another object of the present invention to provide an image forming apparatus that comprises an image bearing body for bearing a toner image; a conveying member for bearing to convey a recording material; and a transferring member for transferring the toner image on the image bearing body to the recording material conveyed by the conveying member by being applied thereon, wherein the transferring member has ion conductivity.
Other objects of the present invention will be apparent from the following description.